PLANAR PROCESSING OF SUSPENDED MICROELECTROMECHANICAL SYSTEMS (MEMS) DEVICES
Suspended microelectromechanical systems (MEMS) devices including a stack of one or more materials over a cavity in a substrate are described. The suspended MEMS device may be formed by forming the stack, which may include one or more electrode layers and an active layer, over the substrate and removing part of the substrate underneath the stack to form the cavity. The resulting suspended MEMS device may include one or more channels that extend from a surface of the device to the cavity and the one or more channels have sidewalls with a spacer material. The cavity may have rounded corners and may extend beyond the one or more channels to form one or more undercut regions. The manner of fabrication may allow for forming the stack layers with a high degree of planarity.
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The present application relates to suspended microelectromechanical systems (MEMS) devices and methods for fabricating such devices.
BACKGROUNDSome microelectromechanical systems (MEMS) devices include a structure suspended over a cavity. For instance, some MEMS resonators include a stack of layers suspended by tethers above a cavity in a substrate. One layer of the stack is sometimes a bottom electrode, another layer is sometimes a top electrode, and a piezoelectric material is between the bottom and top electrodes. MEMS resonators may vibrate at a desired frequency and may be used in frequency and timing applications, including time and clock applications, signal filtering, and motion sensing.
SUMMARY OF THE DISCLOSURESuspended microelectromechanical systems (MEMS) devices including a stack of one or more materials over a cavity in a substrate are described. The suspended MEMS device may be formed by forming the stack, which may include one or more electrode layers and an active layer, over the substrate and removing part of the substrate underneath the stack to form the cavity. The resulting suspended MEMS device may include one or more channels that extend from a surface of the device to the cavity and the one or more channels have sidewalls with a spacer material. The cavity may have rounded corners and may extend beyond the one or more channels to form one or more undercut regions. The manner of fabrication may allow for forming the stack layers with a high degree of planarity.
According to an aspect of the present application, a microelectromechanical systems (MEMS) device is provided. The MEMS device comprises a bottom electrode positioned over a cavity of a silicon substrate, a top electrode, an active layer positioned between the bottom electrode and the top electrode, and at least one channel extending from a surface of the microelectromechanical device to the cavity. One or more sidewalls of the at least one channel include a spacer.
According to an aspect of the present application, a method for forming a microelectromechanical systems (MEMS) device is provided. The method comprises forming a bottom electrode by forming an electrode layer and an active layer over a silicon substrate and patterning the electrode layer and the active layer. The method further comprises forming a top electrode over the bottom electrode and the patterned active layer. The patterned active layer is positioned between the bottom electrode and the top electrode. The method further comprises forming a cavity in the silicon substrate by forming at least one channel that extends from a surface to the silicon substrate and removing at least a portion of the silicon substrate from under the bottom electrode.
According to an aspect of the present application, a method for forming a microelectromechanical systems (MEMS) device is provided. The method comprises forming a bottom electrode, an active layer, and a top electrode of the MEMS device over a silicon substrate. The method further comprises forming at least one channel that extends to the silicon substrate, forming a layer of spacer material on a surface of the at least one channel, and forming a cavity underneath the bottom electrode by removing a portion of the silicon substrate.
Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear.
Aspects of the present application relate to suspended microelectromechanical systems (MEMS) structures, such as MEMS resonators, and to fabrication techniques for making such structures. In some embodiments, the features of the MEMS resonator are largely defined or formed prior to forming the cavity above which the resonator will be suspended. This differs from forming the cavity prior to defining the features of the resonator. Defining the features of the resonator prior to cavity formation may allow for the features to be formed on layers that are more planar than they would be if formed after the cavity. Pre-forming the cavity can create a non-planar surface topology upon which subsequent layers are formed, and this may lead to defects in the subsequently defined features which in turn negatively impact the device performance. Thus, defining the features of the MEMS resonator prior to cavity formation may result in higher performance devices. Such a process may be described as forming the cavity post (after) formation of the resonator stack of layers (referred to as the “resonator stack” for short).
Some aspects of the present application relate to the manner in which the active layer and electrode materials of the MEMS resonator are formed. In some embodiments, the resonator stack may include a bottom electrode, an active layer, and a top electrode, and the bottom electrode and active layer may be formed by depositing and patterning layers of appropriate material prior to formation of the top electrode over the active layer. A cavity may subsequently be formed under the stack after formation of the top electrode. In some embodiments, an oxide layer may be formed over the top electrode and may act to provide thermal compensation to the MEMS device. In some embodiments, the resulting suspended structure may be enclosed by one or more spacer materials (e.g., oxide) in a cross-sectional plane of the MEMS device.
In addition to providing a planar structure while depositing one or more layers of the stack, the fabrication techniques according to aspects of the present application for forming a suspended MEMS structure may reduce cost since a single substrate (e.g. silicon wafer, silicon-on-insulator wafer) can be used.
Fabrication of a suspended MEMS structure by forming a cavity post stack formation may be applied to various types of suspended MEMS structures, including MEMS resonators, accelerometers, and gyroscopes, by forming and patterning different types of materials in the stack. Although much of the discussion herein refers to a suspended MEMS resonator as an example, it should be appreciated that these techniques may be suitable for formation of cavities in a substrate to suspend devices other than resonators.
A suspended MEMS structure fabricated using the techniques described herein may have one or more physical features resulting from the fabrication process. One such feature is the presence of a spacer material (e.g., oxide) on the sides of channels that extend towards the underlying cavity. To form the cavity in the substrate after defining the resonator features, channels may be formed from an upper surface of the substrate to a point deeper than the resonator. Subsequent deposition of layers on the resonator structure, such as oxide layers, may result in such layers covering the sidewalls of the channels. This may be considered a sidewall spacer material in some embodiments. The spacer material may act to reduce or prevent etching of one or more materials of the resonator stack while etching of the substrate to form the cavity.
One or more structural features of the resulting cavity may arise depending on the type of process used to remove the substrate from underneath the stack of materials. To form the cavity, an etching process that removes the substrate horizontally as well as vertically (e.g., an isotropic etch process) may be used to form the cavity after the stack of materials are formed over the substrate. Accordingly, the resulting cavity may have physical features that differ from that of a cavity formed using a vertical etching process (e.g., an anisotropic etch process). In embodiments where an isotropic etching process is used to form the cavity, the cavity may include one or more undercut regions that extend from underneath the stack. The undercut regions may have rounded corners and the curved profile of the rounded corners may depend on one or more conditions of the etching process and/or substrate material. This is contrast with cavity formed in a substrate using an anisotropic etching process, which would result in a cavity having box or square corners.
Another structural feature of a MEMS resonator according to embodiments of the present application is that the top electrode of the resonator may contact the active layer material only in the electrode regions. On the tether regions suspending the resonator body from the substrate, the top electrode material may be separated from the active layer material by a spacer layer, such as an oxide. This may prevent undesired excitation of an active layer at the tether region.
Resonator structure 102, which may be considered as a resonator stack, may include bottom electrode 108, active layer 110, and top electrode 112. Bottom electrode 108, active layer 110, and top electrode 112 are positioned over cavity 104, and active layer 110 is positioned between bottom electrode 108 and top electrode 112. Bottom electrode 108 and/or top electrode 112 may contact active layer 110 at one or more locations. For example, top electrode 112 may have multiple regions in contact with active layer 110, shown in
Resonator structure 102 may include one or more layers of silicon 114. The one or more silicon layers 114 may provide a desired distance between one or more components of the resonator stack (e.g., bottom electrode, active layer, top electrode) from cavity 104. The thickness of the one or more silicon layers 114 may be in the range of 2 microns to 15 microns, or any value or range of values within that range. A silicon layer of region 114 of resonator structure 102 may result from fabrication of suspended MEMS device 110 using a silicon wafer. In such embodiments, a portion of a silicon layer of the wafer may form part of region 114 within resonator structure 102. In some embodiments, a silicon-on-insulator wafer is used to fabricate suspended MEMS device 110, and a layer of silicon over an insulator layer of the wafer may be a silicon layer 114 of the resonator structure 102.
In some embodiments, the one or more layers or silicon 114 may include a layer of epitaxial silicon. The epitaxial silicon layer may be formed on a silicon-on-insulator wafer using a suitable epitaxy deposition process (e.g., chemical vapor deposition). The epitaxial silicon layer may provide a desired thickness of silicon over an insulator layer (e.g., oxide layer) of the silicon-on-insulator wafer. The thickness of the layer of epitaxial silicon may be in the range of 4 microns to 15 microns, or any value or range of values in that range. In some embodiments, the epitaxial silicon layer may be formed over a silicon layer of a wafer (e.g., a silicon-on-insulator wafer). In such embodiments, the silicon layer of the wafer and the epitaxial silicon layer may form the one or more silicon layers 114 shown in suspended MEMS device 100.
Fabrication techniques of the present application may include forming a cavity in a substrate by an etching process that removes the substrate both laterally and vertically (e.g., an isotropic etch process). A cavity formed using the techniques described herein may have rounded corners as a result of lateral etching of the substrate, such as cavity 104 shown in
Another feature of the resulting cavity is that it may extend in a lateral direction of the substrate beyond the suspended structure. In some embodiments, the cavity may extend to another region of the MEMS device, such as a region that includes a contact for an electrode of the MEMS device, forming an undercut region of the cavity. Undercut regions 124a and 124b of cavity 104 extend beyond resonator structure 102 in the cross-sectional plane of suspended MEMS device 100 shown in
Resonator structure 102 may include one or more spacer materials (e.g., oxides). A spacer may act to isolate and/or protect a region of resonator structure 102. In some embodiments, the material used as a spacer may act to provide thermal compensation to suspended MEMS device 100.
In some embodiments, a resonator structure may be enclosed by one or more spacer materials in a cross-sectional plane of the suspended MEMS device. In the cross-sectional plane of suspended MEMS device 100 shown in
Spacer material may be present on other surfaces of a channel that are separate from a suspended structure. Spacer region 118a may be formed as a surface of channel 150a opposite to spacer region 116d, and spacer region 118b may form a surface of channel 150b opposite to spacer region 116e. Spacer regions 118a and 118b may include one or more oxides, or any suitable type of material. In this manner, channels 150a and 150b extend from a surface of MEMS device 100 to cavity 104 and have one or more sidewalls having a spacer material. In some embodiments, one or more sidewalls of a channel extending from a surface of MEMS device 100 to cavity 104 may include an oxide.
A suspended MEMS device fabricated according to the techniques described herein may include a region separated by a channel from the suspended structure. Such a region may be considered as a tether region of the MEMS device. The tether region may have an active layer and a conductive layer (e.g., electrode layer) separated by a layer of spacer material (e.g., oxide). The conductive layer and the active layer may not contact in a tether region of a suspended MEMS device. In some instances, the layer of spacer material may electrically insulate the conductive layer from the active layer. This may prevent undesired excitation of the active layer. As shown in
The present application relates to fabrication techniques used for forming a suspended MEMS device, such as MEMS device 100 having resonator structure 102 shown in
At act 240, a top electrode may be formed over the patterned active layer. The top electrode may be formed using any suitable lithographic techniques. In some embodiments, the method may include forming one or more layers of spacer material over the bottom electrode and the patterned active layer. Formation of the one or more layers of spacer material may include planarization of the spacer material to form a planar surface. Planarization of the spacer material may reduce or remove topological features that can arise from the patterned active layer and the bottom electrode. In some embodiments, one or more layers of spacer material formed over the patterned active layer may be removed to expose regions of the patterned active layer. One or more top electrode regions may be formed in the exposed regions of the patterned active layer. In some embodiments, the method may include forming one or more contacts to a region of the bottom electrode layer. The one or more contacts may electrically couple with the bottom electrode of the resulting MEMS device and may provide and/or receive an electrical signal to the bottom electrode.
At act 250, one or more channels may be formed to extend to the underlying substrate. One or more layers of spacer material and/or silicon may be removed to expose a region of the underlying substrate as part of forming a channel. The one or more channels may separate the patterned active layer and bottom electrode from other regions of the device (e.g., bond pad region, electrode contact region). At act 260, a layer of spacer material may be formed on the sidewalls of the one or more channels. At act 270, a cavity may be formed in the underlying substrate by removing a portion of the underlying substrate from under the patterned active layer and bottom electrode. The layer of spacer material along the sidewalls of the one or more channels may act to protect one or more layers of the suspended structure during formation of a cavity in the underlying substrate. The cavity may be formed using an isotropic etch process, and the cavity may have undercut regions that extend beyond the one or more channels. The cavity may have rounded corners.
Methods of fabricating suspended MEMS devices according to the techniques described herein that implement formation of a cavity underneath a structure stack may reduce the number of process steps needed to form the resulting device in comparison to fabrication methods where the structure is formed over a pre-formed cavity. In some embodiments, the number of resist masks needed throughout the fabrication process may be 5, 6, 7, 8, or any other suitable number. This is in contrast to using a method that forms a structure over a pre-formed cavity where 18 or more resist masks can be used. By reducing the number of steps and/or materials used to fabricate a suspended MEMS device, costs and/or time may be reduced for a suspended MEMS device fabricated using cavity formation post stack formation in comparison to methods that include formation of a suspended structure by forming the structure over a pre-formed cavity.
One or more top electrode regions may be formed by removing oxide in one or more regions over the active layer and forming top electrode regions to contact the active layer.
The resist regions may be removed and one or more top electrode regions may be formed using any suitable deposition process. In some embodiments, a contact for the bottom electrode may be formed during the same stage of the fabrication process as the formation of the one or more top electrode regions.
One or more layers of spacer material (e.g., oxide) may be formed over one or more top electrode regions.
One or more channels may be formed by forming one or more resist regions on a surface opposite the substrate and removing layers of oxide and/or silicon to expose the underlying substrate. The one or more channels may be formed in regions of the surface that are non-overlapping with the patterned active layer and the bottom electrode layer.
A layer of spacer material may be formed on sidewalls of the one or more channels. The layer of spacer material may include one or more oxides, or any other suitable material. In some embodiments, the layer of spacer material may be formed on the sidewalls of the one or more channels and over a silicon layer of the substrate. Oxide over the silicon layer of the substrate may be removed using a suitable etching process to expose the silicon layer before forming a cavity in the silicon layer. The layer of spacer material may act to protect one or more layers, including a silicon layer, the active layer, the bottom electrode, and/or the top electrode, during formation of a cavity in the underlying substrate.
The one or more channels may be positioned to allow for removal of a portion of the substrate underneath the patterned active layer and bottom electrode to form a cavity. The cavity may be formed using any suitable isotropic etch process that etches both vertically and laterally. In some embodiments, an isotropic silicon dry etch process may be used to form the cavity in a silicon substrate. Vapors used during the silicon dry etch process may include xenon difluoride (XeF2) and/or sulfur hexafluoride (SF6).
In some embodiments, a cavity for a suspended microelectromechanical systems (MEMS) device may be confined to desired dimensions or contours within a substrate. In some such embodiments, the confinement may be provided by one or more trenches that extend towards and within the substrate of the device, and which in some embodiments are filled with a material resistant to the etching process used to form the cavity in the substrate. For example, in some embodiments one or more trenches formed in a substrate may confine the extent to which the cavity extends horizontally during an isotropic etch process. In this manner, the one or more trenches may allow for formation of a structure suspended over a cavity with one or more laterally defined dimensions. In some embodiments, the one or more trenches may reduce or prevent the formation of undercut regions during formation of a cavity.
Depending on the type of MEMS device, a cavity confined by one or more trenches may provide desired performance characteristics of the MEMS device, for example by providing a known volume of vacuum or pressurized space in the vicinity of a moving component of the MEMS device. In some embodiments, the one or more trenches may enclose a circumference of the cavity to form a closed contour. The resulting cavity may have a more rectangular cross-sectional shape than a cavity formed without the one or more trenches. In some embodiments, a cavity defined by one or more trenches may have square corners.
The one or more trenches may be formed of any suitable material. In some embodiments, a trench may include one or more oxides. The trench formed of one or more oxides in a silicon substrate may reduce or prevent the formation of undercut regions during etching of the silicon substrate to form a cavity in the substrate. In some instances, a trench formed of one or more oxides may be undesired because some oxides may have material limitations (e.g., tensile stress) that can impact the quality and/or performance of the trench. Some embodiments may have a trench lined with one or more oxides and filled with a suitable filler material (e.g., polysilicon). The oxide may serve to resist etching during formation of the cavity underlying a suspended MEMS device, and the filler material may be provided to reduce material stresses.
Formation of a cavity defined by one or more trenches, as shown in
In some embodiments, MEMS devices of the types described herein may include a cap. For example, a dummy cap, or a cap with integrated circuitry may be used to cap a suspended structure of a MEMS device.
The suspended MEMS devices described herein may be used in various applications and for fabrication of different types of devices that have a structure suspended over a cavity. MEMS devices that may be formed according to the techniques described herein may be applied to the fabrication of resonator MEMS devices, accelerometer MEMS devices, and gyroscope MEMS devices. For example, resonator MEMS devices may be used in frequency sensing applications including gas sensors, humidity sensors, and pressure sensors. The frequency at which the resonator structure resonates may vary depending on the surrounding environmental conditions. In a gas sensor, gas molecules may interact with a surface and the resonator may detect a frequency indicative of the presence of the gas molecules. As another example, resonator MEMS devices may be used in timing applications. For instance, the frequency that the resonator resonates may be used as clock signal, which may be used in the timing of another device or component.
Other uses of the suspended MEMS devices described herein are also possible, as the examples described are non-limiting.
As described, some aspects may be embodied as one or more methods. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.
The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value.
Claims
1. A microelectromechanical systems (MEMS) device comprising:
- a bottom electrode positioned over a cavity of a silicon substrate;
- a top electrode;
- an active layer positioned between the bottom electrode and the top electrode; and
- at least one channel extending from a surface of the microelectromechanical device to the cavity, wherein one or more sidewalls of the at least one channel include a spacer.
2. The MEMS device of claim 1, wherein the bottom electrode, the top electrode, and the active layer are enclosed by at least one spacer material in at least one cross-sectional plane of the microelectromechanical device.
3. The MEMS device of claim 1, wherein the MEMS device further comprises a layer of spacer material positioned over the top electrode, wherein the layer of spacer material is in contact with the top electrode and the active layer.
4. The MEMS device of claim 3, wherein the layer of spacer material is configured to provide thermal compensation for the MEMS device.
5. The MEMS device of claim 1, wherein the cavity extends from underneath the bottom electrode beyond the at least one channel.
6. The MEMS device of claim 5, wherein the cavity has rounded corners.
7. The MEMS device of claim 1, wherein the MEMS device further comprises at least one silicon layer positioned between the bottom electrode and the cavity, and a layer of spacer material positioned between the bottom electrode and the at least one silicon layer.
8. The MEMS device of claim 1, wherein the bottom electrode and the top electrode are in contact with the active layer.
9. The MEMS device of claim 1, wherein the bottom electrode, the active layer, and the top electrode form a resonator region of the MEMS device, and the MEMS device further comprises a tether region separate from the resonator region comprising at least one oxide layer positioned between a layer of active material and a contact, wherein the at least one oxide layer is in the same plane as the active layer of the resonator region.
10. The MEMS device of claim 9, wherein the contact is a contact of the bottom electrode.
11. The MEMS device of claim 1, wherein the MEMS device further comprises at least one trench that extends into the silicon substrate, and the cavity extends to the at least one trench.
12. A method for forming a microelectromechanical systems (MEMS) device comprising;
- forming a bottom electrode by forming an electrode layer and an active layer over a silicon substrate and patterning the electrode layer and the active layer;
- forming a top electrode over the bottom electrode and the patterned active layer, wherein the patterned active layer is positioned between the bottom electrode and the top electrode; and
- forming a cavity in the silicon substrate by forming at least one channel that extends from a surface to the silicon substrate and removing at least a portion of the silicon substrate from under the bottom electrode.
13. The method of claim 12, wherein the method further comprises forming a layer of epitaxial silicon on a surface of the silicon substrate and forming an oxide layer over the layer of epitaxial silicon, and wherein forming the bottom electrode further comprises forming the bottom electrode in contact with the oxide layer.
14. The method of claim 12, wherein the method further comprises forming a layer of spacer material over the top electrode, wherein the layer of spacer material is in contact with the top electrode and the active layer and is configured to provide thermal compensation to the top electrode.
15. The method of claim 12, wherein the cavity extends beyond the at least one channel and has rounded corners.
16. The method of claim 12, wherein the silicon substrate is a silicon-on-insulator substrate, and forming the at least one channel further comprises forming the at least one channel to an insulator layer of the silicon-on-insulator substrate.
17. A method for forming a microelectromechanical systems (MEMS) device comprising:
- forming a bottom electrode, an active layer, and a top electrode of the MEMS device over a silicon substrate;
- forming at least one channel that extends to the silicon substrate;
- forming a layer of spacer material on a surface of the at least one channel; and
- forming a cavity underneath the bottom electrode by removing a portion of the silicon substrate.
18. The method of claim 17, wherein the method further comprises forming a layer of epitaxial silicon on a surface of the silicon substrate and forming an oxide layer over the layer of epitaxial silicon, and wherein forming the bottom electrode further comprises forming the bottom electrode in contact with the oxide layer.
19. The method of claim 17, wherein the method further comprises forming a layer of spacer material over the top electrode, wherein the layer of spacer material is in contact with the top electrode and the active layer and is configured to provide thermal compensation to the top electrode.
20. The method of claim 17, wherein the cavity extends beyond the at least one channel and has rounded corners.
Type: Application
Filed: Nov 28, 2016
Publication Date: May 31, 2018
Patent Grant number: 10800649
Applicant: Analog Devices Global (Hamilton)
Inventors: Michael John Flynn (Waterford), Paul Lambkin (Carrigaline), Seamus Paul Whiston (Limerick), Christina B. McLoughlin (Crecora)
Application Number: 15/362,296